1. Field of the Invention
This invention relates to integrated circuits incorporating MOSFET (metal oxide semiconductor field effect transistor) devices and has particular reference to means for regulating the supply voltage therefor.
2. Description of the Prior Art
Field effect transistors of the MOS type are particularly desirable for computer applications and the like because of the large number of circuits that can be incorporated on a single substrate. One practical limitation, however, in the number of circuits or packing density which can be provided in a given area is the matter of voltage supply regulation. If the voltage supply can be maintained at an optimum level for the threshold voltage of MOSFETs in a particular integrated circuit or "chip", a maximum number of circuits can be incorporated therein. Otherwise, the individual components of the MOSFETs must be made large enough to accommodate correspondingly greater variations in supply voltage. For example, MOSFETs with a relatively small width to length ratio, i.e. 0.1, must be made longer and those with a relatively large width to length ratio must be made wider.
It has been found that the threshold voltage, (the point at which current flows in an enhancement mode MOSFET device as the bias voltage to the gate is increased) is determined by various factors resulting particularly from the manufacture of the device. For example, the threshold voltage may be determined by the temperature, the depth of the various diffusions, the concentrations of additives, impurities, and imperfections in the silicon crystal forming the substrate of the device. Although such threshold point remains fairly constant throughout any one integrated circuit or "chip", it may vary from chip to chip and often requires discarding of certain chips when the threshold point thereof is outside acceptable limits.
Therefore, the supply voltage of the chip should ideally be adjusted in accordance with the threshold voltage of the circuit thereon so as to remain at an optimum level, regardless of ambient temperature, etc.
MOSFET automatic voltage regulators have been used heretofore as exemplified by the U.S. Pat. No. 3,508,084 to R. W. Warner Jr. However, such regulators are employed in the normal manner to supply a constant voltage to a separate load circuit.